For multi-WAN operation with failover, some firewall software (notably pfSense 2.0.1) needs to be able to ping a different IPs for each WAN link. This is particularly true where each upstream WAN link has the same next-hop IP (81.187.81.187).
As a workaround, it would be dead handy if your upstream router 81.187.81.187 would also respond to pings on a few extra IP addresses. Could you make 81.187.81.188, .189, .190 (etc) pingable?
At the moment I'm pinging a few random things like DNS servers, but it would be more reliable to ping the core router itself - without which nothing would work anyway.
Many thanks